The present invention relates to a method for cleaning a vulcanization mold to conveniently remove elastomer residues inevitably formed on inner molding surfaces of a mold (surfaces including mating mold surfaces for a split type mold) and in recessed portions or holes of the mold when the mold is repeatedly used for vulcanization molding of rubber articles such as rubber tires and vibration-proof rubbers. More plastic elastomer articles; and more particularly this invention relates to a method for cleaning a vulcanization mold which is able to stably, uniformly treat the residue without involving any inconvenient degradation or damage of the vulcanization mold and without being affected by formation of a non-uniform zone or region of a plasma distribution. The invention also relates specifically as to a microwave plasma generator which is particularly adapted for cleaning a vulcanization mold and with which the vulcanization residue can be beneficially removed.
As described in detail in Japanese Patent Laid-open No. Hei 6-285868, assigned to the same assignee, in order to satisfy required performance of elastomer articles, particularly, rubber tire articles (hereinafter, referred to simply as tire or tires), and vibration-proof rubber articles, it is necessary that natural rubber, synthetic rubbers or blends thereof be formulated, aside from a sulfur crosslinking agent and a carbon black reinforcing material, with vulcanization accelerators and various types of chemicals for keeping durability.
For vulcanization molding of the thus formulated, unvulcanized rubber composition, chemical reactions including a crosslinking reaction are generally caused at a high temperature close to 200.degree. C. In this condition, the rubber composition not only becomes more fluid, but also is partly gasified. Eventually, it is inevitable that the rubber composition and chemical reaction products fixedly deposit as a residue, although small in amount, not only on the molding surfaces of the mold, but also in very narrow spaces of the mating mold surfaces and in holes such as so-called vent holes whenever the vulcanization molding is carried out. The repetition of a great number of the vulcanization molding cycles permits the residue to be deposited in a thickness which is not negligible. This has been, more or less, experienced in the vulcanization molding of other types of elastomers as well as rubber compositions.
The thick vulcanized residue strongly attached to and deposited on the vulcanization mold impedes the appearance of tires, adversely influences the quality of the tire as a whole. Accordingly, it is necessary that a vulcanization mold after having been subjected to a predetermined number of vulcanization molding cycles is cleaned until it looks like a fresh one. The cleaning has been predominantly performed according to a shot blast method wherein granules such as plastic beads or glass beads are blown against the mold by means of a high pressure gas or according to a liquid cleaning method wherein the mold is immersed in solutions of acids, alkalis or amines. In order to overcome the disadvantages of these cleaning methods, the present applicant has proposed in the above-mentioned Japanese Patent Laid-open No. Hei 6-285868 a vulcanization mold cleaning method using a plasma, with which remarkable results have been obtained.
However, when the results are checked in more detail, it has been found that there is some room for improvement with respect to the following points.
The first point resides in that a multitude of projections such as ribs and sipes (fine elongated pieces) are formed on mold-forming faces of a vulcanization mold to be cleaned in order to form thick grooves, fine grooves and slits in a tread portion which are usually essential to ensure the required characteristics of tires other than specific types of tires. A plasma is interrupted with these projections, so that it is liable to impede uniform ashing of the vulcanized residue over an entire region to be cleaned.
The present applicant has proposed apparatus and method of the uniform ashing of residues in Japanese Patent Laid-open Nos. Hei 8-207056, 8-216164 and 8-244041: one of electrodes to which high frequency electric power is applied is arranged in the form of a cylinder or is arranged as having a multitude of fins on the periphery thereof; and a plasma is caused by discharge between the one electrode and the inner surface of a vulcanization mold serving as the other electrode.
The following points are disadvantageous ones caused by positioning the vulcanization mold within a discharge zone. More particularly, the second point resides in that a non-uniform discharge region is often formed, with the tendency that a uniform ashing treatment is apt to be impeded.
The third point is that a phenomenon sometimes takes place wherein the temperature of the vulcanization mold reaches, for example, 200.degree. C. owing to the non-uniform discharge. This impedes the accuracy of the mold, bringing about the degradation and damage of the mold.
The fourth point has the relation with the third point and resides in the difficult temperature control of the vulcanization mold.
The fifth point resides in that surfaces other than those to be cleaned are exposed to the discharge zone or region, so that the thus exposed surface may be degraded or damaged.
In order to overcome the problems involved in the first to fifth points, we have proposed in Japanese Patent Laid-open No. Hei 8-300366, a method for ashing an elastomer residue wherein a plasma generating furnace capable of supplying neutral active species (radicals), and a microwave generator for converting a reaction gas passed into the furnace to a gas containing neutral active species by means of a microwave discharge plasma are provided in a vacuum treating vessel. The neutral active species generated in the plasma generating furnace are introduced via a pipe into the treating vessel. The thus introduced neutral gas species are uniformly injected toward all faces of a residue on or in a vulcanization mold, thereby ashing the elastomer residue by means of the thus injected neutral active species-containing gas.
The cleaning of the vulcanization mold by the above method shows a remarkable effect on the solution of the problems. Through the cleaning operations of the above method, a further problem to solve has been found. More particularly, the above method is disadvantageous in that a gas which contains a great amount of the neutral active species generated in the plasma generating furnace is fed via a pipe to the treating vessel, during which part of the neutral active species is deactivated, resulting in the lowering of an ashing efficiency.